This invention relates to a variable capacity wobble plate compressor for compressing refrigerant used in air conditioners for vehicles, etc.
A variable capacity wobble plate compressor is known which has such a construction that pressure in a crankcase in which a wobble plate is accomodated is regulated to adjust the inclination angle of the wobble plate, whereby the delivery quantity or capacity varies. This kind of conventional variable capacity wobble plate compressor is disclosed, for example, in Japanese Provisional Patent Publication (Kokai) No. 58-158382.
In this kind of compressor, the pressure in the crankcase where the wobble plate is provided is formed by pressure leaking through clearances between cylinders and pistons, i.e., blow-by gas pressure. Therefore, the pressure in the crankcase is higher than that in the suction chamber of the compressor during operation. When the compressor is heavily loaded due to a great thermal load on the air conditioner, i.e., suction pressure is higher than a predetermined value, a pressure-control valve provided across a communication passage between the crankcase and the suction chamber opens to allow the pressure in the crankcase (formed by blow-by gas) to flow into the suction chamber, whereby the pressure in the crankcase is lowered, accompanied by an increase in the inclination angle of the wobble plate. The increase in the inclination angle of the wobble plate causes a corresponding increase in the stroke of the piston, i.e., an increase in the delivery quantity or capacity. If the thermal load is decreased (medium load state), and accordingly the suction pressure is lowered, the degree of opening of the pressure-control valve becomes smaller in accordance with the lowering of the suction pressure. This reduces the amount of pressure flowing from the crankcase into the suction chamber, which results in an increase in the pressure in the crankcase, accompanied by a decrease in the inclination angle of the wobble plate. This, in turn, shortens the stroke of the piston, i.e., the capacity is decreased. In the above-described manner, the capacity of the compressor automatically varies with a change in the thermal load.
In the meanwhile, when the compressor is under a low load state, if refrigerant is allowed to constantly flow at a fixed rate through the evaporator in spite of a low thermal load, the suction pressure so decreases that the boiling temperature of the refrigerant lowers, which can result in freezing of the evaporator.
In order to prevent such freezing of the evaporator, conventionally, there is provided an antifreeze valve (hereinafter referred to as the A.F. valve). The A.F. valve comprises a piston valve of a check valve type provided across a communication passage which communicates between the discharge pressure chamber and the crankcase. The A.F. valve does not open and is maintained in a closed state when the discharge pressure is in a region higher than a predetermined value. When the discharge pressure is lowered from this region to a freezing region lower than the predetermined value, the A.F. valve opens and part of the pressure in the discharge pressure chamber is allowed to flow into the crankcase, which increases the pressure in the crankcase, accompanied by a decrease in the inclination angle of the wobble plate. This shortens the sroke of the piston, i.e., the capacity is reduced. The reduced capacity lessens the amount of refrigerant passing through the evaporator, which prevents the evaporator from being frozen.
However, if the amount of refrigerant passing through the evaporator is decreased as described above, assuming that the heat exchange rate is constant, the temperature on the low pressure side of the evaporator is raised. The refrigerant gains pressure if its temperature is increased. Thus, if in an attempt to decrease capacity, higher pressure is introduced into the crankcase to make the crankcase pressure higher, the suction pressure is also raised. Then, in response to the elevated suction pressure, bellows of the pressure-control valve provided for keeping the suction pressure constant operates to open the valve, which allows the pressure introduced via the A.F. valve into the crankcase from the discharge pressure chamber to leak into the suction chamber via the pressure-control valve. As a result, the pressure in the crankcase lowers to increase the inclination angle of the wobble plate. Thus, in spite of the A.F. valve being open, the inclination angle of the wobble plate is increased, and part of the compressed refrigerant which is introduced via the A.F. valve into the crankcase from the discharge pressure chamber returns to the suction chamber, which causes power loss.